Field of the Invention
The invention relates to a method and a device for acoustic modulation of a flame through the use of an acoustically modulated feeding of a fuel, wherein the flame is produced by a hybrid burner including a premixing burner and a pilot burner associated with the latter, by burning the fuel which is fed to the premixing burner in a main flow and to the pilot burner in a pilot flow.
The invention relates, in particular, to a method and a device used in a gas turbine in order to affect combustion in the gas turbine with the hybrid burner. That influence takes place, in particular, in order to suppress combustion oscillations in the gas turbine. Combustion oscillations in general are acoustic oscillations which are excited by the combustion itself in a combustion system that is capable of acoustic oscillations. Combustion oscillations should certainly be distinguished from acoustic noise, which can be observed in any sort of combustion system. There is scarcely any flame which is relevant on a technical scale from which no clearly perceptible noise emanates. In contrast with noise, which of its nature contains acoustic signals with frequencies on a comparatively large bandwidth, a combustion oscillation is characterized by a very limited frequency spectrum. In a combustion system in a gas turbine, a combustion oscillation usually has a frequency between approximately 10 Hz and a few kHz. Since a combustion system is frequently of very complicated construction in an industrial system, it is virtually impossible to forecast combustion oscillations with respect to their occurrence and with respect to their frequencies. As a rule, such a forecast already fails because it is impossible to detect the limits of that oscillatory structure which determines the frequency of the oscillations. Consequently, people of relevant experience and responsibility depend on experiments and other empirical measures in avoiding combustion oscillations in combustion systems.
A hybrid burner of the present decisive type is disclosed in European Patent EP 0 193 838 E1 and in ASME publications 90-JPGC/GT-4 and 94-GT-46. The ASME publications also contain many references to the operation of the hybrid burner. The hybrid burner includes a premixing burner, that supplies the predominant component of the heat which can b e produced through the use of the hybrid burner, and in which the fuel and the air, provided for combustion, are intensively mixed with one another, before combustion is initiated. The combustion proceeds with excess air, that is to say the air provided contains more oxygen than is required for complete combustion of the fuel. It may be noted that that is a characteristic of any industrially important combustion system, in particular the combustion system in gas turbines. A pilot burner is provided in the hybrid burner in order to stabilize the combustion proceeding with excess air. The pilot burner is configured in such a way that it operates stably under all conceivable conditions and is constructed, in particular, as a diffusion burner in which the fuel does not mix with air at a substantial distance in time before the actual combustion. Rather, the oxygen penetrates from the air through diffusion into the finely distributed fuel, and the resulting mixture is ignited without a substantial time delay. The pilot burner supplies only a subordinate component of the heat to be produced overall, and thus at most contributes partially to undesired emissions of the hybrid burner, for example nitrogen oxides, irrespective of its construction and mode of operation. It is of principal importance that the pilot burner initiates and stabilizes combustion of the mixture emerging from the premixing burner, and ensures complete combustion of the fuel supplied under all conceivable operating conditions.
European Patent Application EP 0 601 608 A1 and the following articles: "Initiation and Suppression of Combustion Instabilities by Active Control", by T. Poinsot et al., in the 22.sup.nd International Symposium on Combustion, Seattle, Wash., 1988, and the Journal of Propulsion and Power 6 (1990) 324, by P. J. Langhorne et al., provide references to how it is possible for a flame supplied by a burner to be modulated as desired with regard to suppressing combustion oscillations or with regard to other purposes, as well as references to devices and systems which can be appropriately used. A common feature of all of the known methods for modulating a flame, including the "method for active oscillation control" is that those methods have been developed only with an evaluation of the results of experiments on a system set up on a laboratory scale, but not on actual industrial combustion systems. One of the known methods provides for an acoustic wave in a combustion chamber to be extinguished by overlapping an appropriately generated antiphase wave of equal frequency by acoustic irradiation of the combustion chamber with the aid of suitable loudspeakers. Another known method employs modulation of the liquid or gaseous fuel fed to the combustion system. Such modulation is possible, since a combustion oscillation is characterized by a simultaneous and coordinated occurrence of an acoustic oscillation and an oscillating release of heat in the flame. Since the flame supplies the energy for maintaining the acoustic oscillation, a suitable modulation of the fuel flow reaching the flame, and thus of the flame itself, can be used to influence the acoustic oscillation, in particular to damp it. The modulation can be performed, for example, in such a way that the fuel is fed to the flame in an oscillating manner. An actuator which is suitable therefor is described in the above-mentioned European Patent Application EP 0 601 608 A1.
A particular problem arises if the quantity of fuel fed to a combustion system per unit of time is comparatively high, as is the case, for example, with a stationary gas turbine for outputting mechanical power of between 100 MW and 250 MW. Under some circumstances, the modulation of a fuel flow of a corresponding size can require an actuator of a size which is not currently available commercially.